ISIL terrorists in Syria have “publicly executed and crucified” eight rival insurgents fighting the Damascus government as fierce infighting among foreign-backed insurgents in the key Arab country continues unabated, an opposition-linked monitoring group said.

The gruesome development was revealed Sunday by London-based Syrian Observatory for Human Rights, which added that the executions were carried out Saturday in an area east of the Aleppo province.

"ISIS executed eight men in Deir Hafer in the east of Aleppo province," said the Observatory, which is affiliated with Western-sponsored insurgents operating in Syria to topple the Government of President Bashar al-Assad.

ISIS then "crucified them in the main square of the village, where their bodies will remain for three days," the UK-based group further added.

It also explained that the execution occurred because the victims belonged to rival insurgent groups that had also been involved in battling the ISIL elements inside Syria.

ISIS and ISIL are alternative references for the notorious terrorist group that calls itself ‘Islamic State of Iraq and the Levant (greater Syria),’ a radical group that operates in Iraq and Syria and is widely believed to be sponsored by Saudi Arabia and its allied Persian Gulf kingdoms.

Also in Aleppo province, the Observatory added, a ninth man was crucified for eight hours as a form of punishment in Al-Bab town near the border with Turkey, but eventually survived the ordeal.

According to the Western-backed group, ISIL terrorists first emerged in Syria's foreign-sponsored insurgency war in spring of 2013 and was initially welcomed by some of the other armed insurgency groups operating in Syria, believing that its combat experience would help topple the Syrian government.

It also states that rival militants launched a major anti-ISIL offensive in January 2014, and have pushed them out of large swathes of Aleppo province and all of Idlib in the northwest.

However, it notes, ISIL “remains firmly rooted in Raqqa,” its northern Syrian headquarters, and “wields significant power in Deir al-Zor” in the east near the border with Iraq.

The Observatory also expresses concerns that ISIL’s Iraq offensive and capture of heavy, US-made weaponry “appears to have boosted its confidence in Syria.”